1. Field of the Invention
The invention relates to a method for pulling a silicon single crystal from a melt which is contained in a crucible.
2. Background Art
In pulling a silicon single crystal from the melt, a seed crystal is immersed into the melt and raised, material of the melt being crystallized on the seed crystal to form a single crystal. After a stabilization phase, first a neck section is pulled and later a conical section is pulled, the diameter of the growing single crystal being widened to a setpoint diameter during pulling of the conical section. The melt not only contains silicon, but conventionally also one or more dopants which are intended to be incorporated in the lattice of the single crystal in order to determine the electrical conductivity of the single crystal. The melt furthermore contains oxygen, which is dissolved from the crucible material and partially incorporated into the single crystal. The oxygen contained in the single crystal has advantageous and disadvantageous effects.
Precipitates containing oxygen, i.e. so-called BMDs (bulk micro-defects), act as internal getters which can bind metallic impurities and thereby prevent them from interfering with the functions of electronic components. High oxygen concentrations, on the other hand, also promote the formation of detrimental stacking faults, so-called OSFs (oxidation-induced stacking faults). It is therefore desirable to control the oxygen concentration in the single crystal, and likewise the concentration of dopants, so that they do not depart from a narrowly specified concentration range at least in the region of the single crystal's cylindrical section which is used for the production of semiconductor wafers, has the setpoint diameter, and is adjacent to the conical section.
The oxygen concentration in the single crystal can be influenced in various ways, for example by controlling the gas pressure in the reactor, by controlling the speed with which inert gas is fed through the reactor, by controlling the distance between the melt surface and the lower end of a heat shield enclosing the single crystal, by controlling the rotation speed of the single crystal and the crucible, and by controlling the strength of a magnetic field applied to the melt. Despite the use of such control measures, there are problems in obtaining the oxygen concentration in the specified concentration range from the start, i.e. at the beginning of the cylindrical section of the single crystal. If the semiconductor wafers obtained from the start of the cylindrical section of the single crystal do not have the oxygen concentration specified by the customer, this leads to a yield loss which is commensurately more serious when the diameter of the single crystal is larger and when its cylindrical section is shorter.